Editor's note: This report is based on a one-year investigation by CNN into air cargo security in light of a thwarted plot by al Qaeda in October 2010 to blow up cargo jets over the United States. CNN's Nic Robertson's report "Deadly Cargo" aired on CNN Presents in February 2012.

Ibrahim al-Asiri is the sort of terrorist who keeps intelligence officials awake at night. He’s al Qaeda’s chief bomb-maker, and he built explosive devices hidden in printer cartridges that got onto several planes in October 2010. He’s still at large in Yemen. The bomb plots he’s alleged to have masterminded – the 2009 underwear bomb plot and printer bombs dispatched to the United States in 2010 – have very nearly worked. And security experts say al-Asiri and al Qaeda in Yemen may yet penetrate the security screening that is meant to protect aviation.

In the summer of 2009, two Saudi brothers clasped each other in a last embrace in the desert. The elder brother, Ibrahim al-Asiri, had constructed a bomb like none al Qaeda had produced before: a device designed to be inserted inside the rectum of a suicide bomber containing around 100 grams of PETN, a difficult-to-detect white powdery explosive.

The suicide bomber was his younger brother, Abdullah al-Asiri. And their target was Prince Mohammed bin Nayef, the head of Saudi counter-terrorism, whose security services had driven them out of Saudi Arabia two years earlier. Their group - al Qaeda in the Arabian Peninsula (AQAP) - was determined to show that even well-protected targets outside Yemen were not beyond their reach. In the end, the attack - in August 2009 - failed. Despite gaining entry to bin Nayef’s residence by claiming to be defecting, the device killed only Abdullah al-Asiri and slightly injured the head of Saudi counter-terrorism. But even in failure, his brother and comrades were emboldened. Never had al Qaeda come so close to killing a member of the Saudi royal family.

At about the same time, a young Nigerian - Umar Farouk AbdulMutallab - arrived in Yemen. Schooled in the West and from a prominent family, he had become radicalized as a student in London. He was the ideal candidate to carry out AQAP's most ambitious attack yet.

According to recently released court documents, al-Asiri was instrumental in developing plans for the Nigerian to bring down a U.S. passenger jet as it approached its destination. According to the documents, al-Asiri worked in tandem with American-Yemeni cleric Anwar al-Awlaki, who persuaded the Nigerian to conduct a martyrdom mission and approved al-Asiri’s plan for getting a bomb past security onto a U.S.-bound airliner.

Al-Asiri met with AbdulMutallab several times and personally delivered his ingenious new device: 200 grams of PETN stuffed into the lining of specially tailored underwear. According to the court documents, al-Asiri trained AbdulMutallab, "having the defendant practice the manner in which the bomb would be detonated, that is, by pushing the plunger of a syringe, causing two chemicals to mix, and initiating a fire (which would then detonate the explosive)."

AbdulMutallab slipped through airport security to board a Detroit-bound flight on Christmas Day but failed to fully detonate the device as the plane came in for landing. It was a lucky escape. An explosives expert told CNN that one of the likeliest explanations for the failure was the wear and tear on the device during AbdulMutallab's three-week journey through Africa, before flying to the United States.

After the failed attack, the FBI found al-Asiri’s fingerprints on the underwear device and matched them to a file kept by the Saudi government, but the bomb-maker continued to elude the grasp of counter-terrorism agencies.

The following year, Ibrahim al-Asiri began developing his most ingenious device yet. It involved placing 400 grams of PETN inside a printer cartridge and connecting it to a detonator and timer embedded in the circuitry of a laser printer. The choice of a laser printer was deliberate: PETN is similar in texture to ink-toner powder and would therefore evade detection by single-view X-ray machines at many air cargo departure points.

"Whoever designed this is at the clever end of the scale," Sidney Alford, one of the world’s leading explosive experts, told CNN.

Al-Awlaki again played a significant role in the plot. He “certainly encouraged, supported, supervised al-Asiri's efforts," a senior U.S. counter-terrorism official told CNN.

In late October 2010, two printer bombs designed by al-Asiri were dropped off at FedEx and UPS offices in Sanaa, Yemen. They passed through airport security undetected and were then loaded onto the first leg of their journey toward the United States. Only an intelligence tip to Saudi authorities allowed authorities in Dubai and the UK to eventually intercept the deadly cargo. Al-Asiri had concealed the explosives so well that bomb disposal teams at both locations initially believed the printers were not bombs. It was the most sophisticated al Qaeda device that Western counter-terrorism officials had ever seen, and they said it had the potential to bring down a plane.

Al Qaeda later boasted in its online magazine Inspire: "The following phase would be for us to use our connections to mail such packages from countries that are below the radar and to use similar devices on civilian aircrafts in Western countries."

A bomb-maker’s journey

Though al-Asiri remains a shadowy figure, CNN has pieced together details of his journey to jihad. This account is based in part on a detailed briefing on AQAP that Saudi counter-terrorism officials, including Prince Mohammed bin Nayef, provided late last year to Mustafa Alani, the director of security and defense studies at the Gulf Research Center.

Saudi Arabia is generally viewed as having the most extensive intelligence presence in Yemen. Bin Nayef, whose father is crown prince, responded to the assassination attempt against him in 2009 by expanding the Saudi intelligence-gathering operation in Yemen, developing a network of informants, according to Alani. In late October 2010, it was a communication from an informant close to AQAP’s inner circle that tipped Saudi Arabia off to the fact that explosive packages had just been dispatched by the group to the United States, according to Alani.

Al-Asiri was born in Riyadh, Saudi Arabia, on April 18, 1982. His father had served as an officer in the Saudi military, and according to interviews the family later gave the Saudi newspaper Watan, nothing about Ibrahim and his brother Abdullah’s upbringing marked them for jihad.

"They were not religious boys at the time. They used to listen to music and had a wide variety of friends; friends not like the ones they had later when they became more religious," his mother told Watan.

One of their sisters told the newspaper that the death of their brother Ali in a car accident in 2000 was a turning point in Ibrahim and Abdullah's attitude.

"It was after that that they started swapping videotapes and cassettes on the Mujahedeen in Chechnya and Afghanistan, and they became at times distant," the sister said. "Abdullah started to go out a lot with his new friends to camps known as 'preaching camps.’ "

Al-Asiri began studying chemistry at King Saud University in Riyadh but dropped out after only two years, according to Alani. Though he would acquire bomb-making expertise later on, those studies would lay a foundation for his future terrorist career.

Then came the Iraq war. Like hundreds of other young Saudis, al-Asiri was determined to fight in Iraq against the U.S. occupation. But he never made it there. In 2006, he was arrested by Saudi security forces as he tried to cross the border into Iraq. "He was not considered an important person, so he was released after spending a brief amount of time in prison," Alani told CNN. He was held for nine months.

When he was released, al-Asiri, who became known as Abu Salah in militant circles, attempted to create a new militant cell inside Saudi Arabia, linked to al Qaeda, that planned to bomb oil pipelines in the country, according to his later designation as a terrorist by U.S. and U.N. authorities. When police swooped in on their meeting place in northern Riyadh, six of his cell were killed in a shootout, but he and his brother were not there. They were not then viewed by Saudi authorities as key members of the Saudi wing of al Qaeda, according to Alani.

In 2007, al Qaeda in Saudi Arabia instructed its operatives to move to Yemen, according to Alani. Al Qaeda’s Yemen operations had been given a new life after several of its leaders had escaped from prison the previous year. Al-Asiri, then on the run, called his father to tell him he was leaving the country but did not reveal where he was heading. Saudi counter-terrorism were eavesdropping on the call.

The brothers crossed the border into Yemen. Saudi counter-terrorism officials believe that it was only when he moved to Yemen that al-Asiri developed his bomb-making expertise. Alani says there are indications he was tutored by a Pakistani bomb-maker linked to the group.

By the summer of 2009, Ibrahim al-Asiri was one of several Saudis in AQAP’s inner circle. In the weeks leading up to the plot to kill bin Nayef, he and his brother were filmed sitting in a tent with several senior AQAP operatives, including its Yemeni leader, Nasir al-Wuhayshi, a former personal secretary to Osama bin Laden. Some counter-terrorism officials believe that al-Wuhayshi might become the overall head of al Qaeda if Ayman al-Zawahiri is killed. The film was for a propaganda video for the forthcoming attack on bin Nayef.

According to Alani, the most influential figure within AQAP has been another Yemeni - Qasim al-Raymi - who Saudi counter-terrorism officials suspect steered the group toward directly attacking the United States. After his brother’s death, al-Asiri was deployed to work with a group separate from the rest of AQAP and tasked with plotting attacks against the United States, according to Alani. U.S. officials believe that al-Awlaki led the unit.

An enduring threat

A U.S. missile strike in September 2011 killed al-Awlaki.

U.S. officials believe his death has temporarily lessened the threat of an attack on the United States. But they also believe that AQAP has emerged as the most dangerous part of the al Qaeda terrorist network.

As for al-Asiri, "he is in fact undoubtedly one of, if not the largest threats that we face right now," a senior U.S. counter-terrorism official told CNN. "He's smart, determined and quite secretive about his activities and clearly determined."

Saudi counter-terrorism officials believe that political turmoil in Yemen is allowing AQAP to gain strength, according to Alani, because the regime of President Ali Abdullah Saleh has focused its efforts on survival rather than counter-terrorism.

Recent months have seen jihadist militants linked to AQAP but operating under the banner Ansar al Shariah periodically take control of towns in southern Yemen.

In their public statements, AQAP commanders have claimed to be at the forefront of such efforts - in line with their pledge after the death of bin Laden to follow the guidance of al Qaeda’s new leader, al-Zawahiri, whose strategic maxim for jihadists has long been to create "an Islamic base in the heart of the Arab region."

Some eyewitness accounts report a new focus within the group on seizing territory. Abdul Razzaq al-Jamal, a Yemeni journalist who was given unique access to al Qaeda fighters in Abyan province, wrote in the Al Quds al Arabi newspaper last autumn that the group had "used a new strategy recently, which is the strategy of showing themselves and controlling."

Counter-terrorism analysts disagree on how significant a role the group has played in the fighting in Yemen. An extensive field study published by West Point’s Combating Terrorism Center in September found that most of AQAP’s fighters - in the low hundreds - were drawn from urban areas, and there was no conclusive evidence that the group had yet won the allegiance of tribes in southern Yemen.

By contrast, al-Jamal, the Yemeni journalist, described seeing significantly greater numbers of al Qaeda fighters and witnessing their control of several towns in Abyan province last September. Despite a fluctuating situation on the ground, jihadist militants still control significant territory in southern Yemen, including much of the town of Zinjibar, according to reports.

Saudi counter-terrorism officials believe AQAP has taken a back seat in the fighting in Yemen, and has instead taken advantage of the breathing space opened up by jihadist advances to build up its cell structure and a network of safe houses, according to Alani. The group, he says, has learned lessons from Iraq, when seizing territory made al Qaeda an easy target for American airstrikes.

"Their objective in Yemen is to secure a safe haven for recruitment, training and for planning attacks," Alani told CNN.

He says Saudi counter-terrorism officials believe that AQAP's goal is nothing short of eclipsing al Qaeda "central" in the tribal areas of Pakistan as the dominant node of the terrorist network.

"They’ve taken a decision to escalate their global campaign of terrorism," he told CNN. "AQAP believe an attack against the United States is worth a hundred attacks on other places."

Intelligence challenges

The intelligence tipoff that led to the interception of the explosive printers probably saved lives, but it has also made AQAP even more careful about handling information, making it harder for Saudi counter-terrorism to disrupt future plots, according to Alani.

"They tried to find who leaked the information, because the information was so accurate that it must be a human intelligence and not electronic intelligence. Al Qaeda in Yemen became very careful: They hardly use mobile phones, they hardly use any electronic technology."

Saudi Arabia’s counter-terrorism agency believes that al-Asiri has passed on his bomb-making expertise to about five members of the group. "They understand that Asiri is going to be killed or captured one day," Alani told CNN. "We're talking about a new generation of very skillful bomb builders and very committed people."

U.S. counter-terrorism agencies have reached a similar conclusion.

"There are other people who will benefit from his expertise. I think the fear is not just that he'll share his ability within his own circle, but rather more widely, and send it to other al Qaeda-sympathetic individuals or organizations," a senior U.S. counter-terrorism official told CNN.

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CNN's Security Clearance examines national and global security, terrorism and intelligence, as well as the economic, military, political and diplomatic effects of it around the globe, with contributions from CNN's national security team in Washington and CNN journalists around the world.